The present invention relates to an electronic blood pressure meter, and more particularly relates to such an electronic blood pressure meter, the operation of which is, particularly, based upon the oscillation method.
In the prior art, there have been proposed various types of electronic blood pressure meter. Conventionally, a known type of electronic blood pressure meter the operation of which is based upon the oscillation method comprises a cuff which is fitted around the arm of a patient the blood pressure of whom it is desired to measure, a pressurization pump for selectively pressurizing the air which fills a chamber of said cuff (this process will be abbreviated as "pressurizing the cuff" hereinafter), a vent valve for selectively draining said pressurized air from said chamber in said cuff (this process will be abbreviated as "depressurizing the cuff" hereinafter), a pressure sensor for sensing the pressure of said pressurized air in said chamber of said cuff (this will be abbreviated as "cuff pressure" hereinafter) and for outputting a signal (the "cuff pressure signal") representative thereof, and a micro computer or MPU (micro processor unit) for receiving the output signal of said pressure sensor and for calculating a blood pressure value or values according thereto.
Now, the action of such a typical or conventional electronic blood pressure meter will be described in more detail with reference to FIGS. 1(a) through 1(c). These figures relate to a particular typical operational episode in which, after the cuff is fitted around the arm of a patient the blood pressure of whom it is desired to measure, first the cuff is pressurized by operation of the pressurization pump to a pressure substantially greater than the systolic blood pressure of the patient, and then subsequently the cuff is depressurized by operation of the vent valve at a substantially constant rate. FIG. 1(a) shows the behavior with respect to time of the cuff pressure signal, during this operational episode, and illustrates that a pulse wave signal is seen in said cuff pressure signal; FIG. 1(b) shows the behavior with respect to time of the peak amplitudes of this pulse wave signal for each cycle thereof, during this operational episode; and FIG. 1(c) shows the behavior with respect to time of the envelope of said peak amplitudes of said pulse wave signal.
According to the results of various clinical trials, it has been confirmed that the cuff pressure corresponding to the time point, denoted in FIGS. 1(a) and 1(c) as "M", at which the amplitude (denoted as "Ap") of the pulse wave signal reaches its maximum value, denoted in FIG. 1(c) as "Apmax", is a cuff pressure value suitably representative of the average blood pressure of the patient. And, further, according to the results of these clinical trials, it has moreover been confirmed that the cuff pressure corresponding to the time point, denoted in FIGS. 1(a) and 1(c) as "S", at which the amplitude Ap of the pulse wave signal reaches approximately 50% of its maximum value Apmax during increase of said pulse wave signal amplitude with time in the illustrated type of operational episode, i.e. to the left of the point M in FIGS. 1(a) through 1(c), is a cuff pressure value suitably representative of the systolic blood pressure (hereinafter denoted as "SYS") of the patient. And, further, again according to the results of these clinical trials, it has yet moreover been confirmed that the cuff pressure corresponding to the time point, denoted in FIGS. 1(a) and 1(c) as "D", at which the amplitude Ap of the pulse wave signal reaches approximately 70% of its maximum value Apmax during decrease of said pulse wave signal amplitude with time in the illustrated type of operational episode, i.e. to the right of the point M in FIGS. 1(a) through 1(c), is a cuff pressure value suitably representative of the diastolic blood pressure (hereinafter denoted as "DIA") of the patient.
There is however a problem with such an electronic blood pressure meter, in that, the more obese is the patient whose blood pressure is being measured, the flatter is the envelope of the amplitude values of the pulse wave signal, in other words the less are the changes in the amplitude of the pulse wave signal. This is thought to be because an obese patient has a relatively thick layer of fatty tissue under his or her skin, and the pulse wave, which is produced by changes in the volume of the artery or arteries which are obstructed by the constricting action of the cuff, is attenuated by this thick layer of fatty tissue before being transmitted to the cuff.
Now, on the other hand, it is the case that, when the cuff is wrapped around the arm of the patient and is pressurized, a pulse wave signal of a substantially constant amplitude, the so called background pulse wave signal, can always be observed as background to the above described varying pulse wave signal. This background pulse wave signal is generated by a background pulse wave caused by the blood flow in the artery or arteries which is or are obstructed by the constricting action of the cuff, or by minor volume change in the portion or portions of said artery or arteries which is or are closer to the heart of the patient than the portion or portions thereof which is or are being obstructed by the constricting action of the cuff; and said background pulse wave is then transmitted to the cuff and is manifested as a background component to the pulse wave signal which is sensed in the pressure in said cuff by the pressure sensor. And, according to the results of various researches which have been carried out in hospital conditions as well as others by the present inventors, it has been established that the amplitude of this background pulse wave signal is substantially constant irrespective of the actual value of the cuff pressure, and further that the amplitude of this background pulse wave signal does not vary much between one individual and another, whether said individual be obese or otherwise.
In FIG. 1(c) there is shown a case in which said background pulse wave signal has an amplitude denoted as "Ab". In this case, therefore, the true maximum amplitude value of the variable portion of the pulse wave signal, denoted as "Apmg", is equal to the observed maximum amplitude value Apmax of said pulse wave signal, minus this amplitude value Ab of the background pulse wave signal. Now, if the patient is not obese, the maximum observed pulse wave signal amplitude value Apmax is very much greater than the background pulse wave signal amplitude value Ab, and thus the influence from the background pulse wave signal can in practice be neglected. However, if on the other hand the patient is in fact obese, then the maximum observed pulse wave signal amplitude value Apmax is not so very much greater than that the influence of said background pulse wave signal amplitude value Ab that the influence from said background pulse wave signal can be neglected in practice, and problems can arise.
This matter will now be further expatiated upon with reference to FIG. 1(c). The true values for the systolic blood pressure of the patient and the diastolic blood pressure of the patient are obtained, in fact, by calculating as described above based upon the amplitude of the varying portion of the pulse wave signal, in other words based upon the true pulse wave signal amplitude. Thus, considering this true pulse wave signal amplitude which is obtained by subtracting the background pulse wave signal amplitude from the observed pulse wave signal amplitude, as shown in FIG. 1(c) the time point, denoted as "Sg", at which the true pulse wave signal amplitude reaches approximately 50% of its maximum value Apmg during increase of said true pulse wave signal amplitude with time in the illustrated type of operational episode, i.e. to the left of the point M in FIGS. 1(a) through 1(c), is a time value the cuff pressure at which is truly and accurately representative of the true systolic blood pressure (hereinafter denoted as "SYSg") of the patient. And, similarly, the time point, denoted as "Dg", at which the true pulse wave signal amplitude reaches approximately 70% of its maximum value Apmg during decrease of said true pulse wave signal amplitude with time in the illustrated type of operational episode, i.e. to the right of the point M in FIGS. 1(a ) through 1(c), is a time value the cuff pressure at which is truly and accurately representative of the true diastolic blood pressure (hereinafter denoted as "DIAg") of the patient. However, the measured cuff pressure Pc (see FIG. 1(a)) at the time point S derived according to the prior art as first described above and taken as the systolic blood pressure value SYS of the patient, is substantially higher than this true systolic blood pressure value SYSg of the patient. Similarly, the measured cuff pressure Pc (see FIG. 1(a)) at the time point D derived according to the prior art as first described above and taken as the diastolic blood pressure value DIA of the patient, is substantially lower than this true diastolic blood pressure value DIAg of the patient.
These problems are exacerbated, the greater is the value of the background pulse wave amplitude Ab. Thus, if the patient is obese, a large error in measurement may occur, and further the reproducibility of the blood pressure value measurement tends to be deteriorated. In FIGS. 1(b) and 1(c), the broken lines show the pulse wave amplitude values Ap determined in another similar episode of blood pressure measurement under the same or similar conditions. The time points S' and D' are determined by the same method, in this second blood pressure measurement episode, but with regard to the broken line pulse wave amplitude values, as were the time points S and D in the first blood pressure measurement episode, with regard to the solid line pulse wave amplitude values; and it will be seen that these time points S, S' and D, D' are substantially different from one another. Further, in FIG. 1(a), the thus determined systolic blood pressure value SYS' and the thus determined diastolic blood pressure value DIA' corresponding to these time points S' and D' are also shown; and it will be seen that the systolic blood pressure values SYS and SYS', and the diastolic blood pressure values DIA and DIA', are substantially different from one another. Thus a poor accuracy and repeatability of blood pressure measurement are manifested, according to prior art methods as described above.
The more obese is the patient, the more troublesome do the above outlined problems become, and the flatter becomes the envelope of the pulse wave signal amplitude values Ap. Finally, in the case of a quite obese patient, as suggested in the exemplary case of FIG. 1(d), at no point does said pulse wave signal amplitude drop as low as 50% of its maximum amplitude Apmax, and the point S cannot be determined. In an even worse case, at no point would the pulse wave signal amplitude drop as low even as 70% of its maximum amplitude Apmax, and the point D would also become unable to be determined.
Further, another problem is liable to occur with regard to the use of a conventional type of electronic blood pressure meter, which will now be explained with reference to FIGS. 7(a) and 7(b), which relate to the prior art. Namely, it is possible for consistent, i.e. systematic, errors to be generated with respect to accurate blood pressure values as measured by the stethoscopic method.
FIG. 7(a) is a graph showing the correlation between the systolic blood pressure Saus (in mmHg) measured for a patient by using the stethoscopic method, and the systolic blood pressure Sosc (in mmHg) measured for the same patient by using such a conventional type of electronic blood pressure meter which operates according to the oscillation method. The threshold value for the pulse wave amplitude for determining the systolic blood pressure Sosc of the patient, in this exemplary case, was 60% of the maximum pulse wave amplitude. In this exemplary case, the measured systolic blood pressure Sosc tends to be relatively lower for a patient with a relatively high blood pressure and tends to be relatively higher for a patient with a relatively low blood pressure. The points plotted in FIG. 7(a) fall onto the line L's, which is a linear approximation of the correlation between these two systolic blood pressure values Sosc and Saus. This line L's can be expressed by the following linear equation: EQU Sosc=as Saus+bs (a)
The actual values of as and bs were determined, in a typical exemplary case, to be 0.91 (with no dimension) and 11.2 mmHg.
Similarly, FIG. 7(b) is a graph showing the correlation between the diastolic blood pressure Daus (in mmHg) measured for a patient by using the stethoscopic method, and the diastolic blood pressure Dosc (in mmHg) measured for the same patient by using such a conventional type of electronic blood pressure meter which operates according to the oscillation method. The threshold value for the pulse wave amplitude for determining the diastolic blood pressure Dosc of the patient, in this exemplary case, was 70% of the maximum pulse wave amplitude. In this exemplary case, the measured diastolic blood pressure Dosc again tends to be relatively lower for a patient with a relatively high blood pressure and tends to be relatively higher for a patient with a relatively low blood pressure. The points plotted in FIG. 7(b) fall onto the line L'd, which is a linear approximation of the correlation between these two diastolic blood pressure values Dosc and Daus. This line L'd can be expressed by the following linear equation: EQU Dosc=ad Daus+bd (b)
The actual values of ad and bd were determined, in a typical exemplary case, to be 0.91 (with no dimension) and 7.5 mmHg.
In the above description, the correlations between the systolic blood pressure values Sosc and Saus, and between the diastolic blood pressure values Dosc and Daus, were expressed for convenience by linear equations, but in the general case it would be more accurate to express these correlations by quadratic or higher order equations. Generally, these correlations between the systolic blood presure values Sosc and Saus, and between the diastolic blood pressure values Dosc and Daus, can be expressed in the following forms: EQU Sosc=fs(Saus) (c)
and: EQU Dosc=fd(Daus) (d)
and this provides an even more troublesome problem, in the general case.